The molecular function of the auditory and vestibular system in the mouse is the central theme in the Section on Murine Genetics. Spontaneous mutations that cause balance dysfunction and hearing loss are ideal tools to identify genes important for the functioning of the inner ear and to elucidate their role in these sensory systems. Towards this end we concentrated our efforts on three deafness mutations: jackson circler (jc), jerker (je) and Varitint-waddler (Va). By auditory-evoked brain stem response analyses we showed that in these strains hearing impairment is completely penetrant and fully expressed in three to four week old animals. The vestibular phenotype is also fully expressed in je and Va mutants, but seems to vary in jc. To search for hearing and balance modifiers we are currently outcrossing each of the mutations onto different genetic backgrounds. To identify their molecular identity we are in the process of analysing large segregating intersubspecific intercrosses. During the last year we assembled physical BAC clone based contigs for each of the mutated loci and we are in the process of evaluating candidate genes. The molecular cloning of these mutations should give us new insights in the development and function of the mammalian cochlea.